The invention broadly relates to a radiant energy beam scanning system. More particularly, the invention concerns a method and means for generating timing and power level reference signals responsive to the detection of the center of the power distribution within the beam at a predetermined point along a scanned path.
In conventional light beam recording devices, an intensity modulated light beam (e.g. a laser) is repetitively scanned across the surface of a photosensitive medium to produce an imagewise rendition of the desired information. The cyclical scanning pattern is typically established with the use of a moving beam deflector, such as a rotating polygonal mirror or an oscillating galvanometer mirror.
For quality image resolution, the recorded characters are usually formed as composite images of individual picture elements generated on consecutive scan lines. To avoid blurred or deformed characters, these individual picture elements must be aligned, from scan to scan, in the vertical, or cross-scan, direction (i.e., orthogonal to the direction of scan). Various forms of detectors have been proposed heretofore to minimize the effects of this form of image deformation, commonly referred to as jitter. Normally such detectors are positioned at the start of each scan line at a predetermined distance from the edge of the recording surface. A signal generated by the detector is utilized as a timing, or synchronizing, signal to assure that the modulation of the beam begins in precisely the same vertical plane of the beam relative to the edge of the recording surface.
In most commercially feasible systems, however, such detectors do not totally compensate for jitter introduced by irregularities in the beam deflecting mechanism, such as the lack of reflective or surface flatness uniformity between adjacent facets of the polygonal mirror, rotational asymmetry associated with the polygon drive mechanism, and the like. While it is possible to correct for such unavoidable jitter by manufacturing the polygon and drivers to strict tolerances, such measures are generally cost prohibitive for commercial printers.
The foregoing effects are compounded in the systems which employ a solid state diode laser as the recording beam. As is quite well known, the power output of such lasers varies both spatially and in amplitude over time. Conventional beam detectors do not compensate for these variations and, hence, unavoidably produce jitter when utilized in a diode laser environment. Exemplary of these known detectors are the so called slit detectors, which compare the amplitude of a photodiode output signal against a predetermined, fixed reference voltage. When the amplitude of the diode signal passes through this reference threshold, an indicator signal is generated. With a diode laser scanning system, the intensity distribution pattern of the formed beam is generally gaussian. The signal generated by the detector diode will track, in amplitude, this gaussian shape as the beam sweeps across the face of the detector. The outputs produced by beams having different power levels will, necessarily, pass through the fixed reference level at different relative times, resulting in the generation of indicator signals at different points in time in relation to the time reference base of the sweep of the beam. Since the synchronization of the scanner system is keyed to the time difference between the generation of the indicator signal and the transit time of the beam from the detector location to the targeted edge of the recording medium, this differential triggering effects a translation of information horizontally, or in the scanned direction, from line to line so that the picture elements do not align properly in the cross-scan direction. These effects are present for all fixed reference single edge detectors, whether triggered by the leading or trailing edge of the diode output signal.
A known means for overcoming the problem associated with comparing against a fixed reference, is the so called split detector. Such detectors utilize a two photodiode-dual comparator configuration to compensate for variation in beam output power. In operation, the sweep of the beam over the first detector sets an associated first comparator. The output of this first comparator is supplied as a reference for the second comparator, which is thereafter triggered by the sweep of the beam across the second diode detector to provide the indicator signal. While this form of detector performs quite satisfactorily, there is an attendant cost tradeoff.